1. Field of the Invention
This invention relates generally to apparatuses for microfouling control in heat exchangers in a cooling water system of an electric power plant. More particularly, the present invention relates to an improved apparatus and method of removing microfouling from the waterside of a heat exchanger on a more efficient and effective basis.
2. Description of the Prior Art
As is generally known in the electrical power industry, large heat exchangers or condensers are used to condense steam which has been generated in boilers and passed through turbines. Typically, cooling water from a lake or river is drawn by a pump and is continuously passed through an array of sealed tubes of the heat exchanger, and the steam is directed to flow around and between the tubes of the cooling water. As a result, the steam is condensed to water. However, the cooling water contains microorganisms which thrive in the warm environment of the condenser tubes and tends to adhere to the inside or waterside surfaces of the condenser tubes and subsequently multiply rapidly to give microbial deposits or microbial slime. If this process is permitted to continue, the bore of the condenser tubes will eventually become occluded by a slime film due to the microorganisms' growth defined to be "microfouling" and thus impede the performance of the heat exchanger.
Microfouling is a major problem in power plant cooling. Microfouling can impeded heat exchanger performance in one of two ways. First, it can act as an insulator which increases the shell-to-tube side temperature differential. This reduces the efficiency of the heat exchanger. Secondly, if the slime film growth goes unchecked, it can actually reduce cooling water flow through the tubes and thus again reducing efficiency.
A number of prior art methods currently being used in the industry for heat exchanger microfouling control have generally fallen into one of the following categories:
(a) Oxidizing Biocides:
By far the most widely used method for heat exchanger microfouling control is chlorination. On-line injection of chlorine gas or sodium hypochlorite are the two most common methods of chlorination. At the correct concentration, chlorine is a very effective biocide. It is also relatively inexpensive.
In recent years, however, the U.S. Environmental Protection Agency (USEPA) has put tighter restrictions on the discharged chlorine concentration (usually measured as total residual oxidant or TRO). The tighter limits often limit the dose rate and time below that necessary to kill the microfouling. This is particularly a problem when cooling water background levels of ammonium hydroxide are high. Ammonium hydroxide reacts with the chlorine to form chloroamines which reduces the biocidal effect of chlorine without reducing the TRO. The tighter limits have essentially limited chlorination to a microfouling prevention technology only. Once a heavy microfouling film has formed, it is very difficult to remove it with short doses of low level chlorination.
A couple of methods have been used to reduce the discharge TRO while still maintaining effective microfouling control. Alternative chemicals such as chlorine dioxide and bromine chloride have been used. Sodium bromide has been used in conjunction with chlorine in waters high in background ammonium hydroxide. Targeted chlorination has also been used. This method adds the chlorine locally increasing the concentration through the heat exchanger tubes but still maintaining the lower TRO at the discharge. Both of these methods, however, become more difficult to apply as the EPA continues to ratchet the TRO limit downward.
Dechlorination is a second way that more strict EPA limits for TRO have been met. Sulfur dioxide, sodium bisulfite and sodium metabisulfite are the three most popular dechlorination chemicals used. This requires the expense of an additional chemical feed system, however. It also adds more overall chemical to the environment. Finally, some studies have shown that dechlorination reduces but does not eliminate chlorine by-products, such as trihalomethanes.
Ozonation has been used as an alternative to chlorination. The advantage to this technology is that there are no environmentally harmful by-products released to the discharge. Ozone, however, is about twice as expensive as chlorine to treat potable water. This cost differential substantially increases for the treatment of large surface cooling water flows. Ozone is also difficult to handle and must be generated on site. One further caution is that ozone can oxidize manganese and other inorganic materials that can then deposit in the condenser.
Peroxide is the last oxidizing biocide which could show future promise. It is not currently being used on a large scale basis.
(b) On-Line Mechanical Cleaning Methods:
There are several on-line mechanical cleaning technologies currently being marketed. One method recirculates sponge balls through the tube side of the heat exchanger. The balls brush off the microfouling as they circulate through the tubes. A second method uses brushes which are caged in place at each end of the heat exchanger tube. The brushes move back and forth through the tube each time cooling water through the exchanger is reversed. Again the microfouling is scraped off the tube with each pass. Both of these methods require high capital expenditures to install. Often there is not enough space to install a retrofit system. Debris carried into the heat exchanger waterbox poses another difficulty for these systems as it often blocks tubes from the mechanical cleaners.
Another on-line mechanical method currently marketed is a once-through scraper plug design. The plugs are added to the heat exchanger waterbox intakes while online. After passing through the heat exchanger waterbox, the plugs are collected at the discharge. This method has a relatively high operating cost.
Abrasive cleaning, in which sand, glass beads or other abrasives are introduced into the heat exchanger tubes to remove microfouling, is another microfouling control method. Finally, ultrasonic cleaning has shown some success for small heat exchangers.
(c) Ultraviolet Radiation (UV):
UV has proven an effective chlorination alternative for many applications. UV efficiency, however, is impacted by suspended solids. Filtration is generally required, therefore, prior to UV application. This will generally make UV cost-prohibitive for once-through cooling water systems
(d) Off-Line Mechanical Cleaning:
Scraper plugs, scraper brushes and water guns have been used to remove heat exchanger tube microfouling. These methods require draining the waterbox and individually shooting water or a plug through each tube. Many large heat exchangers such as utility condenser waterboxes have thousands of tubes. Therefore, this is a very labor intensive and time-consuming process.
(e) Water Heat Treatment for Macrofouling Control:
This technology circulates heated water through the heat exchanger to remove the biofouling. It has generally been used to remove macrofouling such as for Zebra Mussel control. It differs from the method described herein in that it does not dry out the biofouling but rather heats it in water above the species tolerance level.
(f) Off-Line Tube Air Drying:
Drying the tube side of the heat exchanger while the unit is off-line has been used hereinbefore to dry out and remove microfouling. This method is the most similar to the method of the present invention described herein. With this method an off-line heat exchanger tube side is dried with ambient air, usually by fans or air movers.
The following differences are noted between this past practice and the method of the present invention described herein:
1. In past practices, the heat exchanger tube sides were only dried during outages when the unit was off-line. This had the following disadvantages: PA0 2. Past practices used only ambient air to dry the waterbox. This takes considerably longer than dehumidified air.
(i) It failed to take advantage of the steam heat load to the outside of the heat exchanger tubes found when the unit is on-line. This heat load plays a significant role in drying the heat exchanger tubes in a shorter period of time. PA1 (ii) Depending on the heat exchanger, taking the unit off-line is generally very expensive.
A prior art search directed to the subject matter of this application in the U.S. Patent and Trademark Office revealed the following U.S. Letters Patent:
______________________________________ 4,302,546 4,686,853 4,531,571 4,703,793 4,552,659 4,997,574 4,631,135 5,276,285 ______________________________________
In U.S. Pat. No. 4,531,571 to Robert D. Moss issued on Jul. 30, 1985, there is disclosed a method for feeding chlorine to a heat exchanger for biological fouling control by targeting the feed to only a few tubes at a time. The assembly is comprised of a manifold surrounded by a seal which directly contacts the condenser tube sheet so as to feed chlorine to only a few selected condenser tubes at a time. The seal serves to restrict the flow of water through the tubes so as to increase the contact time between the chlorinated water and the fouling mass in the tubes. The manifold is driven across the entire condenser tube sheet so that all the tubes are chlorinated for the same duration.
In U.S. Pat. No. 4,552,659 to N. Tabata et al. issued on Nov. 12, 1985, there is disclosed an apparatus for preventing biofouling caused by deposition and propagation of shellfish and algae in a cooling water system, using sea water or river water, in a power plant by periodically feeding ozone at high concentration to the system. An ozonizer is combined with an ozone-adsorbing and desorbing device so as to store ozone by adsorbing of an adsorbent and for a long time at low temperatures and desorbing ozone by periodically sucking at high temperatures if desired, by a water ejector.
There is shown in U.S. Pat. No. 4,631,135 to J. E. Duddridge et al. issued on Dec. 23, 1986, a method for reducing or inhibiting of biofouling by contacting the medium capable of causing biofouling with a support material such as synthetic plastic foam. The biological material is thus caused to form on the support material in preference to a part of the system.
There is shown in U.S. Pat. No. 4,997,574 to N. Sarunac issued on Mar. 5, 1991, a method and system for biofouling control in which chlorine, hot water and/or some other control agent is injected by plural stages into the boundary layer. Chlorine residual, water temperature, or some other respective control parameter is maintained in the boundary layer just upstream of the next injection point.
The other remaining patents listed above but not specifically discussed are deemed to be of general interest and to show the state of the art in microfouling control technologies.
None of the prior art discussed above disclose an apparatus and method of removing microfouling from the waterside of a heat exchanger like that of the present invention. The present invention employs an on-line dehumidification method utilizing low relative humidity air for drying the inside surfaces of the tubes of the heat exchanger while the steam side of the heat exchanger is still in service.